Improvements in and relating to vibration control

ABSTRACT

A machine is on a first side of a plate ( 20 ) and a rod ( 50 ) passes along or through the plate ( 20 ). The plate ( 20 ) is susceptible to vibrations arising from operation of the machine. An active vibration suppressor ( 60 ) is mounted on the rod ( 50 ), and a controller is configured to control the active vibration suppressor ( 60 ) to suppress vibrations of the plate ( 20 ).

FIELD OF THE INVENTION

This invention relates to the field of vibration control. Moreparticularly, the invention relates to active suppression of vibrationscaused by machinery. Still more particularly, but not exclusively, theinvention relates to active suppression of vibrations in vehicles, forexample nautical vessels.

BACKGROUND ART

Machinery typically vibrates as it operates. The vibration can causeproblems, for example mechanical damage, reduced efficiency ofoperation, increased noise and discomfort for persons in the vicinity ofthe machine. For example, in a nautical vehicle such as a ship,excessive vibration from the ship's engines can make travel in the shipuncomfortable.

It is known to use active machinery raft isolation mounts to controlvibrations resulting from the operation of machinery. However, machineryis typically coupled to its environment in ways additional to thecoupling through the machinery's mounting, for example by flexiblecouplings associated with exhaust, cooling, fuel and power supplysystems. Such couplings and other connections provide additional pathsfor vibrations to propagate from the machinery, even if no vibrationswere coupled across the mounts.

Wölfel Beratende Ingenieure GmbH+Co. KG produces an active absorber(ADD.Pipe) for the reduction of vibrations in pipes in piping systems inchemical plants and power plants. Vibrations in such systems aretypically caused by water or other liquid hammers, pressure pulses orother excitations. The ADD.Pipe system is a collar that is clamped ontothe piping system and includes a sensor that measures vibrations in thepipe and linear actuators actively controlled to move reaction masses todamp the vibrations in the pipe. However, the system offers only limitedcontrol, and has the potential to suffer from an effect known aspinning, in which vibration at the point of attachment of the collar isreduced but becomes worse at points elsewhere in the pipeline.

It would be advantageous to provide an apparatus, including an activevibration suppressor, in which one or more of the aforementioneddisadvantages is eliminated or at least reduced.

DISCLOSURE OF THE INVENTION

A first aspect of the invention provides an apparatus including amachine, a rod and a plate, wherein the machine is on a first side ofthe plate and at least part of the rod is on the second, opposite, sideof the plate, and wherein the plate is susceptible to vibrations arisingfrom operation of the machine, CHARACTERISED BY an active vibrationsuppressor mounted on the rod, and a controller configured to controlthe active vibration suppressor to reduce vibrations of the plate.

It may be that the rod passes through the plate, from the first side ofthe plate to the second side of the plate.

It may be that the vibrations of the plate are minimised.

It may be that the machine is a source of motive force, for example anengine, for example an internal combustion engine, an electric motor ora turbine.

It may be that the rod is connected, for example directly connected, tothe machine. It may be that the rod is driven by the machine. It may bethat the rod is also susceptible to vibrations arising from operation ofthe machine.

It may be that the rod is a pipe. It may be that the rod is a cable or abundle of cables. It may be that the rod is connected to or associatedwith the machine, for example it may be a pipe carrying a fluid to orfrom the machine (for example, fuel, coolant or exhaust) or a conduitcarrying for example electrical power couplings.

It may be that the rod is flexible. Alternatively, it may be that therod is rigid.

It may be that the plate is a flat plate. It may be that the plate is acurved plate.

It may be that the plate is a shell or a wall or a bulkhead.

It may be that the plate is or is part of a housing containing themachine. The housing may, for example, be a cuboidal housing, or adome-shaped housing, or have another or a more complex shape. It may bethat the plate forms part of a wall of the housing: the first side ofthe plate will then be inside the housing and the second side of theplate will be outside the housing.

It may be that the plate is part of the machine itself, or is directlyconnected to the machine.

It may be that the rod or at least part of the rod is perpendicular tothe plate.

It may be that the rod or at least part of the rod is parallel to theplate.

It may be that the rod is rigidly connected to the plate.

It may be that the rod is resiliently connected to the plate

It may be that the rod is formed from a first part on the first side ofthe plate and a second part on the second side of the plate, such thatthe first and second parts meet at the plate and are connected to eachother and/or to the plate.

It may be that the active vibration suppressor is mounted on the rod onthe second side of the plate, i.e. the opposite side from the machine.

It may be that the active vibration suppressor includes a collar that isconnected to and at least partially surrounds the circumference of therod. It may be that the collar completely surrounds the circumference ofthe rod. It may be that the collar is integrated with the rod (i.e. theyare monolithic). It may be that the collar is permanently fixed to therod. It may be that the collar is releasably fixed to the rod. It may bethat the collar is moveable on the rod. It may be that the collar ispositioned immediately adjacent to the plate.

It may be that the active vibration suppressor comprises a plurality of(for example 5) actuators arranged to act on the rod. It may be that theactuators act on the rod directly or it may be that the actuators act onthe rod indirectly, for example through the collar, if present. It maybe that at least one of the actuators is arranged to act in a directionparallel to the length of the rod. It may be that there are threeactuators, arranged to act on the rod in three orthogonal directions. Itmay be that two or more of the plurality of the actuators are arrangedto act on the rod the same direction, for example in a directionparallel to the length of the rod. For example, there may be three ormore actuators arranged to act in a direction parallel to the length ofthe rod by acting on a first surface of a collar connected to and atleast partially surrounding the rod, the first surface beingperpendicular to the rod, and there may be two or more further actuatorsarranged to act in two directions orthogonal to the length of the rodand each other by acting on the collar on a surface perpendicular to thefirst surface. It may be that the plurality of actuators includes one ormore actuators arranged to apply a bending moment to the rod. It may bethat the plurality of actuators includes two or more actuators arrangedto apply orthogonal bending moments to the rod. It may be that theplurality of actuators includes one or more actuators arranged to applyradial forces to induce compression in the rod; advantageously, the rodmay be a pipe arranged to carry a flow and the controller may beconfigured to control that one or more actuator to suppress flow noisein the pipe.

The actuators may be magnetostrictive actuators. The actuators may bevoice-coil activators. The actuators may be piezo-electric devices.

The apparatus may further comprises a plurality of sensors arranged tosense vibrations of the plate and to generate a signal indicative ofthose vibrations. The sensors may for example measure the velocity ofthe plate at the location of the sensor. The controller may beconfigured to receive the generated signal and to use it to determine acontrol signal to be generated and applied to the actuators to suppressvibrations of the plate. The controller may be configured to suppressvibrations of the plate by minimising a parameter derived from thesignal from the sensors, for example minimising the sum of mean squaredvelocities measured by the sensors. The controller may be configured tocontrol three degrees of freedom of movement of the plate, for examplemovement in three orthogonal directions, two in the plane of the plateand the third perpendicular to the plate, or one perpendicular to theplate and moments about two perpendicular axes in the plane of theplate. The controller may be configured to control five degrees offreedom of movement of the plate, for example movement in threeorthogonal directions, two in the plane of the plate and the thirdperpendicular to the plate, and moments about two perpendicular axes inthe plane of the plate. It may be that there is the same number ofactuators as there are degrees of freedom of movement controlled by thecontroller.

It may be that the controller is configured to control the activevibration suppressor also to reduce vibrations of a further part of theapparatus, for example a part of the rod or another rod included in theapparatus. The apparatus may then further comprise a plurality ofsensors arranged to sense vibrations of the further part of theapparatus and to generate a signal indicative of those vibrations. Thesensors may for example measure the velocity of the further part at thelocation of the sensor. The controller may be configured to receive thegenerated signal and to use it to determine a control signal to begenerated and applied to the actuators to suppress vibrations of thefurther part. The controller may be configured to suppress vibrations ofthe further part by minimising a parameter derived from the signal fromthe sensors, for example minimising the sum of mean squared velocitiesmeasured by the sensors. The controller may be configured to controlthree degrees of freedom of movement of the further part, for examplemovement in three orthogonal directions. The controller may beconfigured to control five degrees of freedom of movement of the furtherpart, for example movement in three orthogonal directions and momentsabout two perpendicular axes. It may be that there is the same number ofactuators as there are degrees of freedom of movement controlled by thecontroller.

A second aspect of the invention provides a method of suppressingvibrations arising from operation of a machine, wherein the machine ispart of an apparatus also including a rod and a plate, the machine beingon a first side of the plate and at least part of the rod being on thesecond, opposite, side of the plate, the plate being susceptible tovibrations arising from operation of the machine, the method comprisingapplying an actively controlled force to the rod to reduce vibrations ofthe plate.

A third aspect of the invention provides vibration suppression equipmentsuitable for use in reducing vibrations in an apparatus comprising amachine, a rod and a plate, the equipment comprising:

-   -   (i) an active vibration suppressor, adapted to be mounted on the        rod; and    -   (ii) a controller configured to control the active vibration        suppressor to reduce vibrations of the plate.

It will of course be appreciated that features described in relation toone aspect of the present invention may be incorporated into otheraspects of the present invention. For example, the method of theinvention may incorporate any of the features described with referenceto the apparatus of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings, of which:

FIG. 1 is an apparatus that is an example embodiment of the invention,shown in (a) front elevation and (b) side elevation;

FIG. 2 is a front elevation of a collar from the apparatus of FIG. 1;

FIG. 3 is a plot, against frequency in Hertz, of power in Watts inputinto the plate of the apparatus of FIG. 1 in a simulation, withoutcontrol (solid line), when the power input into the plate is minimisedusing three vertical (i.e. z-axis, parallel to the beam) control forces(dashed line) and using three vertical control forces and two horizontal(i.e. parallel to the plate) control forces (dotted line), all acting onthe plate;

FIG. 4 is a plot, against frequency in Hertz, of power in Watts inputinto the plate of the apparatus of FIG. 1 in a simulation, withoutcontrol (solid line) when the power input into the plate (dashed line)is minimised and also when the sum of the velocity squared, measuredusing 3 sensors located on the plate, is minimised (dotted line), wherethe controller consists of three forces acting on the beam at 3 cm fromthe plate;

FIG. 5 is a plot, against frequency in Hertz, of power in Watts inputinto the plate of the apparatus of FIG. 1 in a simulation, withoutcontrol (solid line) when the power input into the plate (dashed line)is minimised and also when the sum of the velocity squared, measuredusing 3 sensors located on the plate, is minimised (dotted line), wherethe controller consists of a vertical force and two moments acting onthe beam at 3 cm from the plate;

FIG. 6 is a plot, against frequency in Hertz, of the sum of the meansquared value of the velocities along the z-axis measured by fivemonitoring sensors in a simulation, without control (solid line) whenthe power input into the plate is minimised (dashed line) and when thesum of the mean squared value of the velocities measured by three errorsensors is minimised (dotted-line), for the same conditions as were usedto generate FIG. 5;

FIG. 7 is a plot, against frequency in Hertz, of the sum of the meansquared value of the velocities in the z-direction measured by (a) themonitor sensors and (b) the error sensors before control (solid line)and after control (dashed line), in a simulation employing transferfunctions measured on the apparatus of FIGS. 1 and 2, and in which threeactuators apply a common vertical force and two further actuators applyhorizontal forces;

FIG. 8 is a plot, against frequency in Hertz, of the sum of the meansquared velocities in z-direction measured by (a) the monitor sensorsand (b) the error sensors before control (solid line) and after control(dashed line), in a simulation identical to that used to generate FIG.7, but in which three actuators apply three independent vertical forcesand two further actuators apply horizontal forces; and

FIG. 9 is a plot of the power spectral density (PSD) of the sum of theaccelerations measured by the three error sensors without control(predominantly upper line) and with control (predominantly lower line),along the (a) x axis, (b) y-axis and (c) z-axis, obtained by experimentwith the apparatus of FIGS. 1 and 2 being driven at 447 Hz.

DETAILED DESCRIPTION

An example embodiment of the invention was created as an experimentaltest apparatus 10 (FIGS. 1(a) and (b)). The test apparatus 10 includes acantilever plate 20 supported on a base sheet 25. The base sheet 25 ismounted on a plurality of isolation mounts 40, which act to isolate theplate 20 from environmental vibrations, to improve the accuracy of thetesting. The cantilever plate 20 is coupled with a pipe 50 having itsaxis perpendicular to the plate 20. The pipe 50 is free at both of itsends 55 a, 55 b. The pipe 50 passes through a hole in the plate 20, thehole being off-centre, offset towards the top and side of the plate 20.

An active vibration suppressor including a moveable collar 60 is mountedon the pipe 50, on the side of the plate 20 towards the left-hand end 55b of the pipe 50, as shown in FIG. 1(b). In this experiment, the collar60 was mounted so that it abutted the plate 20. The collar 60 carried aplurality of actuators 70.

Vibrations were excited in the plate 20 and pipe 50 using a primaryactuator (shaker) 30, mounted on the pipe 50, on the opposite side ofthe plate 20, close to the opposite end 55 a of the pipe 50. This shaker30 was used to excite vibrations representing the vibrations that wouldbe excited by a machine 30′ connected to the end 55 a of the pipe 50.

The collar 60 and actuators 70 are shown in more detail in FIG. 2. Thecollar 60 is a steel annulus 80, approximately 20 cm in diameter with acentral circular hole 9 cm in diameter. The annulus 80 is of rectangularcross-section, having flat front and rear surfaces and flat inner andouter walls. Five actuators 70 a-e are mounted on the annulus 80: threeare mounted on the front surface and two on the outer wall. The threeactuators 70 a-c mounted on the front surface of the annulus 80 arespaced equidistantly around the annulus, i.e. separated by 120 degrees.These three actuators 70 a-c act in a direction perpendicular to theplane of the annulus 80, i.e. parallel to the axis of the pipe 50 whenthe collar is in use. The other two actuators 70 d,e, mounted on theouter wall of the annulus, act in directions perpendicular to the planeof the annulus 80 and perpendicular to each other. Thus the threeactuators 70 a-c on the front surface, on the one hand, and the twoactuators 70 d,e on the outer wall on the other hand together provideactuation in three orthogonal directions.

A computer model of a beam was used to represent the plate 20 and pipe30, with the beam having the same length and second moment of area asthe pipe 30. The geometrical and physical parameters of the system aresummarised in Table 1.

TABLE 1 Parameter Value PIPE Diameter of the pipe cross D = 89.9 × 10−3m section Thickness of the pipe s = 5 × 10−3 m Length of the pipe l = 1m Mass of the pipe Mpipe = 5.3 Kg PLATE Plate's edges (lx, ly) = (1, 1)m Thickness h = 20 × 10−3 m Intersection between the (bx, by) = (375 ×10−3, 1.125 × 10−3) mm plate and beam Mass of the plate Mp = 155 KgMATERIAL STEEL Density ρ = 7750 Kg/m3 Young Modulus E = 200*109 N/m2Poisson's coefficient ν = 0.33

When the model of the system is excited at the end of the beamcorresponding to the end 55 a of the pipe 50 by three in-phase forcesalong each of the three main directions, resonances are clearly evident,in both the beam and the plate.

In the simulations, 285 modes of the flexural vibration of the plate and15 modes of the in-plane vibration of the plate, the flexural vibrationof the beam and axial vibration of the beam were included. In order togain a better understanding of the dynamics of the system the velocitiesin the z direction at 1156 points scattered on the plate were calculatedwhen the system was excited at frequencies that coincide with the firstfew resonances of the system.

Simulations were carried out both of the case in which the activevibration suppressor is placed on the plate 20 and the case in which itis place on the pipe 50.

In the first simulation, a feed-forward control strategy was selected tominimise the power input into the plate, using multiple actuatorslocated on the plate, adjacent to and surrounding the beam. Threecontrol forces were provided in the z direction (parallel to the pipe)and two control forces along the x and y directions. FIG. 3 shows thepower input into the plate without control (black line), when thecontroller is optimised to minimise the power input into the plate ateach frequency when only three vertical forces are used (dashed line),and when five control forces (three vertical and two horizontal) areused (dotted line). The best reduction of the power input into the plateis obtained at low frequency when the wavelength is much bigger than thedistance between the secondary sources and the intersection pointbetween the plate and the beam. The plot shows that the use of two extracontrol forces in x and y directions allows the system to achieve somecontrol even at the in-plane resonance frequency of 1800 Hz.

For simulation of the arrangement in which the control sources areplaced on the pipe 50, the actuators were located in the model on thebeam at 3 cm from the plate, on the unexcited side of the beam.

From a practical point of view, measurement of the power input into theplate 20 would be difficult to implement. An alternative and morepractical control strategy is to measure the velocities at a number ofpoints on the plate 20 and thus to minimise the sum of the mean squaredvelocities.

Three different configurations of the control system were simulated. Inthe first configuration a force acting along the axis of the beam andtwo forces along the x and y axis are used as secondary sources. In thesecond configuration a force acting along the axis of the beam and twomoments around the x and y axes are used as secondary sources. In thelast case three forces and two moments acting on the beam are used assecondary sources.

The model included three error sensors placed on the plate 20, aroundthe pipe 50. The distance between the axis of the pipe 20 and the errorsensor was 20 cm. There were also five monitoring sensors positioned onthe plate 20, which could measure the velocity along the z-axis only(this constraint was used to represent the experimental facility). Thepurpose of the monitoring sensors was to obtain an indication of theglobal response of the plate 20 when control is applied.

For the first configuration, FIG. 4 shows the modelled power input intothe plate without control (solid line), when the optimal controller thatminimises the power input into the plate is implemented (dashed line)and when the sum of the velocities squared measured by three errorsensors is minimised (dotted line). The three error sensors are able tomeasure the velocity along the three principal axis x, y, and z. FIG. 4shows that minimising the sum of the mean squared velocities measured bythe three error sensors closely corresponds to the minimisation of thepower input of the plate at low frequency, although at higher frequencyless reduction in the power input into the plate is obtained in thisexample when the sum of the mean squared velocities is minimised.

The second configuration simulates a vertical force along the z-axis(i.e. along the pipe 50) and two moments around the x- and y-axes actingon the pipe 50. FIG. 5 shows the modelled power input into the platewithout control (solid line), when the optimal controller that minimisesthe power input into the plate is implemented (dashed line) and when thesum of the velocities squared measured by three error sensors isminimised (dotted line). The plot shows that in this case the twocontrol strategies give very similar reduction in the power input intothe plate. Moreover, for this case, a vastly superior reduction comparedwith the previous case of three secondary forces was achieved. The twopeaks that appear around 1.2 and 1.8 kHz are due to flexural vibrationof the beam.

FIG. 6 shows the sum of the mean squared velocities measured by the fivemonitoring sensors without control (solid line), when the power inputinto the plate is minimised (dashed line), and when the mean squaredvelocity measured by the three errors sensors is minimised (dottedline). The plot shows that excellent overall reduction in the vibrationof the plate can be achieved using both control strategies up to about1.8 kHz.

Further simulation results have shown that in the third configuration(i.e. with three secondary forces acting along the x, y and z axes andtwo moments around the x- and y-axes) in theory both the power injectedinto the plate, and the sum of the mean squared velocities are perfectlycontrolled; they are not reproduced here since the results tend to zero.

We now return to the experimental apparatus 10 of FIG. 1. The fiveactuators 70 a-e on the collar 60 are each inertial electromagneticactuators. As discussed above, actuators 70 a-c generate a forceperpendicular to the plate 20 while actuators 70 d and 70 e produceforces along the other two principal directions. The collar 60 can berotated and moved along the pipe 50; however, in the experimentalresults presented below the collar 60 is positioned as close as possibleto the plate 20. As in the simulations, three accelerometers (not shown)are used as error sensors for control purposes; they are able to measurethe acceleration along the three principal axes. Four single axisaccelerometers (not shown) were also placed on the plate 20 in order tomonitor the global response. The primary disturbance excitation wasprovided using a Data Physics IV 40 inertial actuator (shaker) 30located near the tip 55 a of the pipe 50. That primary shaker 30 is alsomounted on a collar which enables a variety of disturbances to beeffected, by mounting the shaker 30 in different orientations.

Before attempting real-time control the level of performance achievablewith this arrangement was assessed. For this purpose the transferfunctions from, on the one hand, the primary source 30 and the actuators70 to, on the other hand, all the sensors mounted on the plate 20 weremeasured. Those measured transfer functions were then used in furthersimulations to predict the behaviour of the system when differentharmonic control strategies were implemented. The measured transferfunctions were also used for the identification of the system requiredto implement real time control. All the transfer functions between theprimary actuator 30 and secondary actuators 70 and the error and monitorsensors were measured using a twenty channel analyser (Data PhysicsMobilyzer II). Although the transfer functions between the primaryshaker 30 and the sensors have been measured for 4 differentorientations of the primary actuator, only the results when the primaryforce forms an angle of about 45 degrees with each of the principal axisare presented here.

FIG. 7 shows the modelled sum of the mean squared monitor velocities(plot (a)) and error velocities (plot (b)) without control (solid line)and when the optimal simulated control is implemented (dashed line). Inthis example, actuators 70 a-c were driven with the same signal tominimise the sum of the three mean squared velocities measured by thethree error sensors along z-axis. Actuators 70 d,e were driven with twodifferent signals to minimise the sum of the mean squared velocities inx- and y-axes respectively. In this configuration, the controller 75applies three orthogonal forces aligned with the principal axes near thepoint of intersection of the pipe 50 and plate 20 and so is not able toproduce moments to control vibration arising from this mode ofexcitation. Nevertheless, the plots show that significant reduction inexcess of 40 dB across a broad range of frequencies can be achieved byimplementing this type of control. The resonance of the primary shakeris about 30 Hz, therefore at lower frequencies the structure cannot beefficiently excited. The resonance of the secondary actuators 70 isabout 55 Hz therefore control will not be effective at frequenciessignificantly below this.

A much better broadband reduction can be achieved when the fivesecondary actuators 70 a-e are driven independently to minimise the sumof the mean squared velocities measured by the three error sensors. Forthis case the controller 75 is able to produce three orthogonal forcesand two moments. FIG. 8 shows the modelled sum of the mean squaredmonitor velocities (plot (a)) and error velocities (plot (b)) withoutcontrol (solid line) and when the optimal simulated control isimplemented (dashed line).

A comparison between FIG. 7 and FIG. 8 clearly shows that the mostefficient control strategy is that used to produce the latter plot, i.e.one able to generate moments and forces as already predicted by thesimulation results from the beam-plate model. It will be noted that theresults indicate that a mean reduction in the region of 50 dB isachievable across a very broad band of frequencies. For some specificfrequencies however, notably around 390 Hz, the reduction is not asgreat. However, that this is not significant since that frequencycorresponds to a structural mode that is not strongly excited by thepipe excitation and could be tackled with a different distribution oferror sensors.

Finally, some preliminary experimental results that demonstrate theperformance and potential of the control system are presented. In thisexperimental work, a controller 75 with only three degrees of freedomwas used, i.e. where the three secondary actuators 70 a-c on the frontface of the annulus 80 were each driven using the same signal tominimise the sum of the mean squared accelerations measured by thecorresponding three error sensors in the z direction and the actuators70 d,e on the side of the annulus 80 were driven with two differentsignals to minimise the sum of the mean squared accelerations along thex- and y-axes respectively. This limited structure was used due toprogramme time constraints and the immediate availability of suitablecontrol software. The harmonic controller 75 was implemented in feedbackmode and the disturbance path is therefore assumed to be unknown. Theprimary disturbance shaker 30 was driven with a sinusoidal signal of 447Hz and oriented to form an angle of 45 degrees with the three principalaxes.

FIG. 9(a)-(c) shows the PSD of the sum of the accelerations measured bythe three error sensors along x, y and z-axis respectively withoutcontrol (blue line) and with control (red line). An iterative frequencydomain algorithm was used for the control strategy; however, in the timeavailable a rigorous search for the optimal parameters was not possibleand so these results do not represent the true optimum performance forthis configuration. Nevertheless, and despite the limited controlauthority, the plots show that the error signals at the excitationfrequency of 447 Hz are reduced by 17.3, 17.2 and 32.5 dB respectively.That clearly demonstrates the validity of the theoretical results aboveand underlines the enormous potential of the approach.

Thus we have shown that an active approach can be used to reduce thelevels of plate vibration caused by the solid penetration of a vibratingpipe by several orders of magnitude. The best achievable performance ofthose investigated here was shown to be obtained by applying controlledforces and moments to the pipe using an actuation system with fivedegrees of freedom. In simulation work based on a rigorous mobilityanalysis a mean suppression in the region of 50 dB was shown to betheoretically achievable for a generic interacting plate-pipe system. Anactuated pipe collar having the necessary five degrees of freedom wasdeveloped and a prototype system was built and trialled using a purposebuilt laboratory scale test rig. This system was tested underconstrained operation, but despite that achieved reductions in anacoustically radiating structural mode of over 30 dB. Use of a collarcarrying the actuators has the particular benefit that a high level ofvibration suppression can potentially be achieved for all transmissionpaths that propagate from a given isolation system (e.g. across flexiblecouplings associated with exhaust, cooling, fuel and power supplysystems), and not merely those propagating through the system'smounting.

Whilst the present invention has been described and illustrated withreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not specifically illustrated herein.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims. Moreover, it is to be understood thatsuch optional integers or features, whilst of possible benefit in someembodiments of the invention, may be absent in other embodiments.

1. An active vibration suppressor apparatus for use with an arrangementincluding a machine, a rod and a plate, wherein the machine is on afirst side of the plate and at least part of the rod is on the second,opposite, side of the plate, and wherein the plate is susceptible tovibrations arising from operation of the machine, the apparatuscomprising an active vibration suppressor mounted on the rod, and acontroller configured to control the active vibration suppressor toreduce vibrations of the plate.
 2. An apparatus as claimed in claim 1,in which the rod passes through the plate, from the first side of theplate to the second side of the plate.
 3. An apparatus as claimed inclaim 1, wherein the rod is connected to the machine.
 4. An apparatus asclaimed in claim 1 wherein the rod is a pipe.
 5. An apparatus as claimedin claim 1 wherein the plate is or is part of a housing containing themachine.
 6. An apparatus as claimed in claim 1, wherein the plate ispart of the machine itself, or is directly connected to the machine. 7.An apparatus as claimed in claim 1 wherein the active vibrationsuppressor is mounted on the rod on the second side of the plate.
 8. Anapparatus as claimed in claim 1 wherein the active vibration suppressorcomprises a plurality of actuators arranged to act on the rod.
 9. Anapparatus as claimed in claim 1 wherein the active vibration suppressorincludes a collar that is connected to and at least partially surroundsthe circumference of the rod.
 10. An apparatus as claimed in claim 9,wherein three or more actuators are arranged to act in a directionparallel to the length of the rod by acting on a first surface of thecollar, the first surface being perpendicular to the rod, and two ormore further actuators arranged to act in two directions, orthogonal tothe length of the rod and each other, by acting on the collar on asurface perpendicular to the first surface.
 11. An apparatus as claimedin claim 1, further comprising a plurality of sensors arranged to sensevibrations of the plate and to generate a signal indicative of thosevibrations.
 12. An apparatus as claimed in claim 11, wherein thecontroller is configured to suppress vibrations of the plate by reducingthe value of a parameter derived from the signal from the sensors. 13.An apparatus as claimed in claim 12, wherein the parameter is the sum ofmean squared velocities measured by the sensors.
 14. A method ofsuppressing vibrations arising from operation of a machine, wherein themachine is part of an apparatus also including a rod and a plate, themachine being on a first side of the plate and at least part of the rodbeing on the second, opposite side of the plate, the plate beingsusceptible to vibrations arising from operation of the machine, themethod comprising applying an actively controlled force to the rod toreduce vibrations of the plate.
 15. Vibration suppression equipmentsuitable for use in reducing vibrations in an apparatus comprising amachine, a rod and a plate, the equipment comprising: an activevibration suppressor, adapted to be mounted on the rod; and a controllerconfigured to control the active vibration suppressor to reducevibrations of the plate.
 16. The vibration suppression equipment ofclaim 15, wherein: the rod is connected to the machine and passesthrough the plate, from the first side of the plate to the second sideof the plate; the plate is part of the machine itself, or is directlyconnected to the machine.
 17. The vibration suppression equipment ofclaim 15, wherein the active vibration suppressor comprises a pluralityof actuators arranged to act on the rod.
 18. The vibration suppressionequipment of claim 15, wherein the active vibration suppressor includesa collar configured to be connected to and at least partially surroundthe circumference of the rod.
 19. The vibration suppression equipment ofclaim 18, wherein three or more actuators are arranged to act in adirection parallel to the length of the rod by acting on a first surfaceof the collar, the first surface being perpendicular to the rod, and twoor more further actuators arranged to act in two directions, orthogonalto the length of the rod and each other, by acting on the collar on asurface perpendicular to the first surface.
 20. The vibrationsuppression equipment of claim 15, further comprising a plurality ofsensors configured to generate a signal indicative of vibrations of theplate, wherein the controller is configured to suppress vibrations ofthe plate by reducing the value of a parameter derived from the signalfrom the sensors, wherein the parameter is the sum of mean squaredvelocities measured by the sensors.